Gravity Assist (Slingshot)

A gravity assist is a technique where a spacecraft flies close to a planet or moon to gain (or lose) velocity relative to the Sun using the body's gravitational field — effectively stealing a tiny fraction of the planet's orbital momentum. Voyager 2 used gravity assists from Jupiter, Saturn, and Uranus to reach Neptune. JUICE (Jupiter Icy Moons Explorer) used a Earth–Venus–Earth–Earth sequence to reach Jupiter. The manoeuvre is "free" in terms of fuel but requires precise timing and trajectory design. In the reference frame of the assisting body, the spacecraft's speed doesn't change — only its direction.